This invention relates to a system for purifying hydrogen and in particular to such a system in which hydride forming material is utilized to purify hydrogen.
Because of the abundance of hydrogen and its relatively pollution-free burning qualities, the desirability of developing hydrogen as an energy source has long been recognized. Perhaps the principal drawback in utilizing hydrogen thus far has been the difficulty of efficiently and safely storing hydrogen. Storing hydrogen as a liquid is costly since it requires considerable energy to liquify the hydrogen, and transfer of the liquid from one container to another results in a loss to the atmosphere of much of the hydrogen. Also, containers for the liquid hydrogen must be extremely well insulated to reduce the loss of hydrogen due to vaporization or boiling. Storing hydrogen as a gas requires extremely heavy and bulky containers and is impractical for most presently contemplated uses.
The use of hydride forming material (hereinafter defined to mean any metals, metal compounds or alloys or other materials capable of adsorbing and holding hydrogen) appears to be an attractive approach to the storage of hydrogen. Exemplary hydride forming material includes iron titanium, misch-metal tetranickel and columbium. Storage of hydrogen in the hydride forming material (to form hydrides by what is sometimes referred to as hydriding) typically involves applying hydrogen gas under pressure to the material and then dissipating the heat generated by the hydriding process. After the material adsorbs the hydrogen, the material is sealed in a container under pressure to maintain the material in the "hydrided" state until the hydrogen is needed at a subsequent time. Recovery or withdrawal of the hydrogen involves a process substantially opposite that used for storing the hydrogen i.e., releasing some of the pressure of the container in which the hydride is maintained.
Hydride forming material presently contemplated for use in storing hydrogen not only adsorbs hydrogen but also some impurity gases, such as water vapor and oxygen, which are generally present with commercial sources of hydrogen. Some impurity gases are more readily released from the hydride forming material than is hydrogen and some are less readily released. If hydride forming material is repeatedly "charged" with hydrogen and the latter type impurities and then only hydrogen is released for use, ultimately the hydride forming material will have adsorbed so much impurity gas that it becomes unsuitable for storing hydrogen. Thus, the simple process of repeatedly applying such impure hydrogen to hydride forming material for storage is undesirable since the material eventually becomes incapable of the desired adsorption of hydrogen.
Another problem of storing hydrogen in hydride forming material arises from the fact that most such material is granulated so that a certain amount of void space is present in the material. This can be advantageous in one respect since it facilitates the flow of hydrogen through the material during the hydriding process, but disadvantageous in another respect since the voids receive and retain the impurities present with the hydrogen which are not adsorbed. Of course, it would be desirable to purge the impurities from the voids prior to supplying the hydrides to users since the presence of such impurities might be detrimental to the intended use.